A deflection optical sensor based on a Scotch tape waveguide with an integrated grating coupler

A deflection sensor based on a plastic waveguide cantilever is presented and demonstrated. The waveguide cantilever is made of conventional adhesive tape and integrates a metal grating coupler at its deflecting end. Light impinging the grating is coupled into the tape waveguide and guided to a fixed photodetector on which the opposite end of the waveguide is anchored. The photodiode acts as the cantilever support and converts guided optical power into a photocurrent (sensor response). Deflection optical monitoring relies on the variation of the overlap of the incident light beam spot with the grating coupler as a function of the cantilever deflection. This approach leads to a larger deflection sensitivity than that obtained by a method based just on the variation of the grating coupling efficiency with the incidence angle. A 14.85-mm-long cantilever sensor has been fabricated and exhibits a linear working range of 2 decades with a maximum deflection sensitivity of 0.2 μA/μm and a resolution of 1.7 μm, limited by the interrogation light source noise. Noise analysis indicates the feasibility of sub-nanometric deflection resolution.